に違いない is used when you are very sure about something, almost to the point of no doubt. It is similar to saying 'must be' or 'surely' in English. You use it when you have good reasons or strong evidence for your conclusion, but you are not stating it as an absolute fact.

This pattern is more common in written Japanese and formal speech. In casual conversation, people often use に決まっている or はずだ instead, which sound more natural. However, に違いない is perfectly acceptable in essays, reports, or when speaking in a serious, formal context.

Note that for nouns and na-adjectives, you need the だ before に違いない. For i-adjectives and verbs, you attach directly to the plain form.